1. Field of the Invention
The invention relates to filter units.
A filter unit contains a filter element and receives fluid to be filtered. After passage through the element, filtered fluid passes from the unit. Filter elements have only a limited life; after a time they become clogged with filtrate and require replacement by removal of a used filter element from the filter unit and the insertion of a new filter element.
In critical production processes, there are often found hydraulic and lubrication systems that are required to operate continuously with extremely clean fluid and so fluid is passed to such systems through a filter unit. When the filter elements in such units become blocked, it is necessary to replace them immediately without interruption of fluid flow, thereby maintaining system availability.
2. Brief Description of the Prior Art
In the past, this has been dealt with in a number of ways. In one proposal, the filter unit has included two filter elements arranged in parallel so that fluid to be filtered is normally passed to one of the elements in the unit but, when that element requires replacement, the fluid is passed to the other of the elements.
In a second but similar arrangement, two filter units are arranged in parallel and pipework and valves are arranged so that fluid to be filtered can be passed through one or other of the units. In the same way as with the first proposal, therefore, one filter element can filter fluid while the other is being replaced.
A third proposal is to provide a single filter unit and to incorporate the unit into pipework that provides a by-pass between the inlet to the unit and the outlet to the unit. By providing a valve in pipework leading to the inlet, a normally closed valve in the by-pass pipework and a non-return valve in pipework leading from the outlet, closure of the inlet valve isolates the unit for replacement of the filter element while the by-pass valve is opened so that the fluid passes through the by-pass during replacement.
The proposals in which two filter units are provided have a disadvantage that two filter elements are needed when only one is required for most of the time. Thus there is capital outlay on filter capacity that is, in fact, only needed very intermittently and for a short time. In addition, the size of such an assembly is substantial and this may make it difficult to install in certain locations.
Where two parallel filter units are provided, the installation and additional pipework and valves may result in large pressure drops across the unit which is disadvantageous. There may also be difficulties in closing off one filter unit before opening the other and this may result in isolation of the downstream system from fluid flow, which is again disadvantageous.
A single system with external valves and pipework has the disadvantage that the additional pipework and valves increase the cost and cause a large pressure drop across the system. Also, it may not be possible simultaneously to open the by-pass valve as the inlet valve is closed (and vice versa) and this may isolate the downstream system from fluid flow.
GB-A-656494 discloses a filter unit including a filter element with an inlet passage leading to the element and an outlet passage leading from the filter element. A by-pass passage extends between the inlet and outlet passages and includes a valve which opens and closes the by-pass passage in accordance with the absolute fluid pressure. The inlet and the outlet passage are controlled by valves responsive to the difference in pressure between the inlet passage and the outlet passage.
U.S.Pat. No. B 4,379,053 discloses a filter unit including a filter element with an inlet passage leading to the filter element and an outlet passage leading from the filter element. A by-pass passage extends between the inlet and outlet passages. A rotary valve in one position diverts incoming fluid into the inlet passage and closes the by-pass passage and in a second position closes the inlet passage and opens the by-pass passage. An anti backflow valve is provided in the outlet passage upstream of its connection to the by-pass passage.